1. Field of the Invention
The present invention relates to a display device, and more particularly, to an organic electroluminescent (EL) display using electroluminescence of organic matter.
2. Discussion of the Related Art
In general, an organic EL display is a displaying device for electrically exciting phosphorous organic compounds to emit light. The organic EL display drives nxm organic light emitting elements arranged in a matrix format to represent images.
The organic light emitting elements have diode characteristics so they may be referred to as organic light emitting diodes (OLEDs), and have a structure including an anode electrode layer (ITO), an organic thin-film layer, and a cathode electrode layer (metallic). The organic thin film has a multi-layered structure including an emitting layer (EML), an electron transport layer (ETL), and a hole transport layer (HTL) to balance electrons and holes and improve light emission efficiency, and additionally has an electron injecting layer (EIL) and a hole injecting layer (HIL). The organic light emitting elements form an organic EL display panel through an arrangement in an nxm matrix format.
Methods for driving the organic EL display panel include a passive matrix method and an active matrix method which uses thin-film transistors (TFTs). The passive matrix method forms anodes and cathodes to cross (or cross over) with or to be substantially perpendicular to each other, and selects lines to drive organic EL elements. The active matrix method sequentially turns on a plurality of TFTs coupled to data lines and scan lines according to scan select signals to thus drive organic EL elements.
A pixel circuit of a general active matrix organic EL display will be described.
FIG. 1 shows one of nxm pixels, that is, equivalently illustrating a pixel provided on the first row and the first column.
As shown in FIG. 1, a pixel 10 has three sub-pixels 10r, 10g, and 10b which have organic EL elements OLEDr, OLEDg, and OLEDb respectively emitting red, green, and blue (RGB) light. In the case of sub-pixels arranged in a stripe pattern, the sub-pixels 10r, 10g, and 10b are coupled to data lines D1r, D1g, and D1b, and a common scan line S1.
The red sub-pixel 10r includes transistors M11r and M12r and a capacitor C1r for driving the organic EL element OLEDr. Likewise, the green sub-pixel 10g includes transistors M11g and M12g and a capacitor C1g, and the blue sub-pixel 10b includes transistors M11b and M12b and a capacitor C1b. The connection and operation of only the sub-pixel 10r will now be described since the connections and operations of the sub-pixels 10r, 10g, and 10b are substantially the same.
The driving transistor M11r is coupled between a power supply voltage VDD and an anode of the organic EL element OLEDr to transmit a light emitting current to the organic EL element OLEDr, and a cathode of the organic EL element OLEDr is coupled to a voltage of VSS which is lower than the power supply voltage VDD. The current of the driving transistor M11r is controlled by a data voltage applied through the transistor M12r. In this instance, the capacitor C1r is coupled between a source and a gate of the transistor M11r to maintain the applied voltage for a predetermined time. A gate of the transistor M12r is coupled to the scan line S1 for transmitting an on/off-type select signal, and a source of the transistor M12r is coupled to the data line D1r for transmitting a data voltage corresponding to the red sub-pixel 10r. 
In operation, when the switching transistor M12r is turned on in response to a select signal applied to the gate, a data voltage of VDATA provided by the data line D1r is applied to the gate of the transistor M11r. A current (IOLED) flows to (and/or through) the transistor M11r in correspondence to a voltage charged between the gate and the source by the capacitor C1r, and the organic EL element OLEDr emits light in correspondence to the current IOLED In this instance, the current IOLED flowing to the organic EL element OLEDr is given in Equation 1.
                              I          OLED                =                                            β              2                        ⁢                                          (                                                      V                    GS                                    -                                      V                    TH                                                  )                            2                                =                                    β              2                        ⁢                                          (                                                      V                    DD                                    -                                      V                    DATA                                    -                                                                                V                      TH                                                                                          )                            2                                                          Equation        ⁢                                  ⁢        1            
where VTH is a threshold value of the transistor M11r, and β is a constant.
As represented by Equation 1, in the pixel circuit shown in FIG. 1, a current corresponding to the data voltage is supplied to the organic EL element OLEDr, and the organic EL element OLEDr emits light with a brightness corresponding to the supplied current. In this instance, the applied data voltage has plural values within a predetermined range in order to represent gray scales.
As described, the organic EL display allows one pixel 10 to have three sub-pixels 10r, 10g, and 10b, each of which includes a driving transistor, M11r, M11g or M11b, a switching transistor, M12r, M12g or M12b, and a capacitor, C1r, C1g or C1b for driving an organic EL element. Also, a data line, D1r, D1g or D1b, for transmitting data signals and a power electrode line for transmitting the power supply voltage VDD are provided for each sub-pixel.
Therefore, the number of transistors, capacitors, and wires for transmitting voltages and signals is increased so that it is difficult to lay out all of them in the pixels, and aperture ratios corresponding to light-emitting areas in the pixels are decreased (i.e., a ratio between the bright pixel area and the pixel area that is blocked by the parts to drive each pixel is decreased).